In some wireless communication systems, neighboring channels or operating bands fall within a duplex filter RF pass-band or transition band of a communication device resulting in signal interference at the device. In FIG. 1, for example, there is only a 5 MHz guard band between the EUTRA Band 33 TDD uplink/downlink (UL/DL) and EUTRA Band 1 FDD UL. There is only a 2 MHz guard band between the Public Safety (PS) narrowband (NB) downlink (DL) frequency band and the EUTRA Band 13 FDD uplink (UL). Similarly, there is potentially only a 5 MHz guard band between the Band 7 FDD UL and the Band 38 TDD UL/DL depending in the specific regulatory and UMTS extension band auction outcomes. Thus a EUTRA UE transmission may interfere with public safety equipment or another EUTRA UE reception operating on a neighboring band, or with UMTS equipment etc.
FIG. 2 illustrates transmissions by user equipment (UE) that adversely affects a co-located UE operating on a neighboring band. For example, the UE may be a EUTRA UE interfering with another EUTRA UE or a UE operating on the public safety band as discussed above. UE transmission interference with neighboring frequency bands depends generally on the output power of the UE, the transmit bandwidth and on the location of the transmit frequency relative to the neighboring band. For example, control channels implemented using narrowband frequency resources located toward the edges of a wideband frequency resource tend to interfere with neighboring bands, particularly at higher transmit power levels. In EUTRA, the PUCCH control channel is located near or at opposite edges of a wideband frequency resource to provide diversity and to avoid fragmentation of the resource block allocation space used for data traffic transmissions. More generally, interference on adjacent bands may also result from spurious emissions due to frequency domain images generated by radio frequency (RF) impairments such as quadrature (I/Q) imbalance imperfections, local oscillator leakage and DC component feed-through, and the associated inter-modulation products (typically 3rd order, but other inter-modulation orders are possible) that fall within the RF pass-band or transition band of the duplex filter.
FIG. 3 illustrates a spectrum model for a 10 MHz EUTRA Channel neighboring a 6 MHz Public Safety Band with a 2 MHz guard band (from −5 to −7 MHz in FIG. 3). The location of the 10 MHz EUTRA channel is shown in FIG. 3 from −5 MHz to 5 MHz with the Public Safety Band located at −7 MHz to −13 MHz with a 2 MHz guard band from −5 MHz to −7 MHz. The spectrum or power spectral density (PSD) due to a 1 Resource Block (RB) (consisting of 12 adjacent subcarriers with a subcarrier spacing of 15 kHz) transmission with maximum transmit power of 23 dBm located near the edge of the 10 MHz EUTRA channel that is closest to the Public Safety Band (e.g., transmission frequency near −4.5 MHz) is shown in FIG. 3. Transmission at the maximum power of 23 dBm corresponds to a Maximum Power Reduction (MPR) of 0 dB. A local oscillator (LO) carrier feed-through (DC component) of −30 dB and I/Q imbalance resulting in a I/Q image (located near +4.5 MHz) with power 30 dB below the desired transmit power is assumed. The 3rd order inter-modulation distortion spurious components between the desired spectral component (located near −4.5 MHz) and its I/Q image labeled “Image spurious” and between the desired spectral component and LO leakage or carrier feed-through labeled “I/Q spurious” are shown in FIG. 3. The PSD of the 3rd order inter-modulation distortion spurious components is a function of RB power and is reduced by 3 dB for each 1 dB increase in MPR. The Image spurious bandwidth is 3 times the desired RB allocation bandwidth which is −0.5 MHz for the case of 1 RB allocation or 180 kHz bandwidth in FIG. 3. The LO or I/Q spurious bandwidth due to 3rd order inter-modulation is smaller. The location of the 3rd order inter-modulation distortion spurious components is a function of the allocation RB location with its position changing depending on the RB offset from the channel edge. It can be seen that the Image and I/Q spurious emissions are significant with no duplex filter. The −30 dBm/100 kHz (−20 dBm/1 MHz) spurious emissions does not meet the UE to UE co-existence target which is typically around −50 to −60 dBm/100 kHz. Even with a typical duplex filter, attenuation will only provide mitigation if spurious emission falls within RF filter stop band, which is typically located at a 6 MHz offset for 700 MHz carrier frequency. In FIG. 3, the duplex filter stopband, shown starting from −12 MHz, provides mitigation of the Image spurious emission only with possibly limited attenuation, if any, of the I/Q spurious emission component. Thus, significant interference to the adjacent Public Safety Band DL band (close geographical proximity between a (EUTRA UE) transmitter and a (PS NB) UE receiver) may occur, requiring techniques for interference reduction and control for neighboring operating bands in wireless communication systems.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.